Photochemical Production and Biological Consumption of Carbon Monoxide (CO) in the Sea Surface Microlayer of Temperate Coastal Waters: Implications for Air-sea CO Exchange

Youta Sugai, Atmosphere and Ocean Research Institute, The University of Tokyo, Marine Microbiology, Department of Marine Ecosystems Dynamics, Kashiwa, Japan, Kenji Tsuchiya, National Institute for Environmental Studies, Center for Regional Environmental Research, Tsukuba, Japan, Shinji Shimode, Yokohama National University, Graduate School of Environment and Information Sciences, Yokohama, Japan and Tatsuki Toda, Soka University, Graduate School of Engineering, Hachioji, Japan
The sea surface microlayer (SML), less than 1,000-µm uppermost layer of the ocean water column, is located at the air-sea interface and plays critical roles in global biogeochemical cycles and climate change through air-sea gas exchange. To clarify the significance of the dynamics of carbon monoxide (CO) in the SML, where active photochemical and biological processes occur, in air-sea CO exchange, the production and consumption of CO in the SML and its sea-air emission were investigated from June 2017 to June 2018 in temperate coastal waters of Sagami Bay, Japan.

In the SML, the light-normalized photochemical CO production rate was relatively high from spring to autumn (3.85 ± 3.09 nM [kWh m–2]–1) when relatively high absorbance of chromophoric dissolved organic matter (CDOM) (0.69 ± 0.38 m–1) was observed. Biological CO consumption rate constant in the SML showed relatively high values from spring to autumn (0.060 ± 0.010 h–1) during the period of relatively high water temperature (22.3 ± 2.7°C). Although the calculated sea-air CO flux (F) varied similarly to CO concentration ([CO]) in the subsurface water, [CO] was particularly high in the SML (15.0 nM) during a phytoplankton bloom there, which indicates the suppression of air-sea gas exchange by the accumulation of biogenic surfactants in the SML.

The turnover time of the photochemical production (τprod), biological consumption (τcons), and sea-air emission (τsea-air) of CO in the SML were compared. τcons (19.0 ± 5.0 h) was about three orders of magnitude higher compared to τsea-air (0.024 ± 0.024 h) during the study period, suggesting that biological consumption in the SML can be ignored in air-sea CO exchange throughout the year. However, although τsea-air/τprod ratio was low most of the year (< 3.03%), τprod was relatively comparable to τsea-air in August 2017 with τsea-air/τprod ratio of 21.9%, which suggests that photochemical production in the SML enhances F during summer.